Various operations can be performed in well, including firing perforating guns, setting packers, and/or actuating other types of downhole tools. A common technique of actuating a downhole tool is by using a hydraulic actuator that employs a delayed trigger mechanism. The delayed trigger mechanism typically includes an orifice through which hydraulic fluid is metered at a relatively slow rate from a high pressure chamber that contains the hydraulic fluid to a low pressure chamber (e.g., air chamber).
The rate at which the hydraulic fluid flows through the orifice provides the delay to allow for delayed triggering when actuating the tool. An issue associated with using such a hydraulic actuator is that a rheological characteristic of the hydraulic fluid may be unstable at different temperatures and pressures, especially elevated temperatures and pressures that are present in a well. This may result in unpredictable delays provided by the delayed trigger mechanism of the hydraulic actuator. As a result, a tool may not exhibit a desired performance due to the instability of the rheological characteristics of a hydraulic fluid.